Evenity (Romosozumab): History & Development

From Rare Bone Disease to Drug Target: The Discovery of Sclerostin

The development of romosozumab traces back to observations in two rare genetic conditions. In the 1950s and 1960s, clinicians described patients with sclerosteosis and van Buchem disease, conditions characterized by dramatically increased bone mass throughout life. These patients had bones so dense they almost never fractured.

The molecular basis remained unknown until 2001, when two independent research groups identified loss-of-function mutations in the SOST gene as the cause of sclerosteosis 1. The SOST gene encodes sclerostin, a glycoprotein produced almost exclusively by osteocytes (mature bone cells embedded within the mineralized matrix). Sclerostin acts as a negative regulator of the Wnt signaling pathway, which controls osteoblast differentiation and bone formation. When sclerostin is absent or reduced, osteoblasts proliferate and form bone at accelerated rates.

This genetic insight provided a clear therapeutic hypothesis. If inhibiting sclerostin caused extreme bone density in rare disease, a drug that partially blocked sclerostin might safely increase bone density in osteoporosis patients. Amgen initiated preclinical development of anti-sclerostin antibodies in the early 2000s, with UCB joining as co-development partner in 2004.

Mechanism of Action: How Romosozumab Works

Romosozumab is a humanized IgG2 monoclonal antibody that binds to and neutralizes sclerostin. This single molecular action produces a dual physiological effect that no prior osteoporosis drug achieved.

By blocking sclerostin, romosozumab disinhibits the canonical Wnt/beta-catenin pathway in osteoblast lineage cells 2. The downstream effects are twofold. First, osteoblast precursor cells differentiate at higher rates, and mature osteoblasts increase their bone-forming activity. Serum markers of bone formation (P1NP) rise sharply within the first month of treatment. Second, romosozumab decreases expression of RANKL by osteocytes, which reduces osteoclast activity and bone resorption. Serum CTX (a resorption marker) drops within the first week.

This "modeling-based" bone formation differs from the mechanism of teriparatide (Forteo), which stimulates both formation and resorption through PTH receptor activation. The net result: romosozumab builds new bone faster than any other approved agent while simultaneously preventing existing bone from being broken down.

One critical pharmacodynamic feature shapes clinical use. The bone formation effect is self-limiting. P1NP levels peak at approximately one month, then gradually return toward baseline by month 9 to 12, even with continued dosing 3. This "anabolic window" is why treatment is limited to 12 monthly doses, after which patients transition to an antiresorptive agent to maintain gains.

Preclinical Development and Early Human Studies (2004 to 2012)

Amgen's preclinical program demonstrated proof-of-concept in multiple animal models. In ovariectomized rats and cynomolgus monkeys, anti-sclerostin antibodies increased bone mineral density at the spine and hip by 20% to 40% within months, far exceeding gains from bisphosphonates or teriparatide in the same models 4.

Phase I studies began in 2007. A single-dose escalation trial in healthy postmenopausal women (N=72) showed dose-dependent increases in bone formation markers and decreases in resorption markers, with BMD gains of up to 5.3% at the lumbar spine within 85 days of a single subcutaneous dose 5. No serious adverse events were identified. The speed of BMD accrual was unprecedented for any osteoporosis therapy.

The Phase II trial (2008 to 2010) randomized 419 postmenopausal women with low BMD to various romosozumab doses or comparators (placebo, alendronate, or teriparatide) for 12 months 6. The 210 mg monthly dose produced 11.3% lumbar spine BMD gain at 12 months, compared to 4.1% for teriparatide and 7.1% for alendronate given sequentially. This dose was selected for Phase III.

The FRAME Trial (2016): Fracture Reduction Against Placebo

The FRAME study (FRActure study in postmenopausal woMen with ostEoporosis) was the first key Phase III trial. Published in the New England Journal of Medicine in 2016, it enrolled 7,180 postmenopausal women with osteoporosis (T-score between -2.5 and -3.5) at sites in Latin America, Central/Eastern Europe, and Asia 7.

Patients received romosozumab 210 mg or placebo monthly for 12 months, followed by denosumab 60 mg every 6 months in both groups for an additional 12 months. At 12 months, romosozumab reduced new vertebral fractures by 73% versus placebo (0.5% vs. 1.8%, P<0.001). The benefit persisted through month 24 after transition to denosumab: 75% reduction in vertebral fractures over the full study period.

Lumbar spine BMD increased by 13.3% at 12 months with romosozumab versus 0% with placebo. Total hip BMD increased by 6.9% versus 0%. These were the largest BMD gains reported for any osteoporosis trial at the time.

FRAME did not demonstrate a statistically significant reduction in nonvertebral fractures at 12 months, though a prespecified sensitivity analysis excluding Latin American sites (where placebo fracture rates were unexpectedly low) showed significance.

The ARCH Trial (2017): Head-to-Head Against Alendronate

The ARCH trial (Active-contRolled fraCture study in postmenopausal women with osteoporosis at High risk) compared romosozumab directly against alendronate, a standard-of-care bisphosphonate. Published in the New England Journal of Medicine in 2017, it randomized 4,093 postmenopausal women with osteoporosis and a prior fragility fracture 8.

Patients received either romosozumab 210 mg monthly or alendronate 70 mg weekly for 12 months, then both groups transitioned to open-label alendronate. Results were definitive. At 24 months, romosozumab followed by alendronate reduced new vertebral fractures by 48% compared to alendronate alone (6.2% vs. 11.9%, P<0.001). Nonvertebral fractures were reduced by 19% (9.7% vs. 13.0%, P=0.04). Hip fractures were reduced by 38% (2.0% vs. 3.2%, P=0.02).

Dr. Felicia Cosman, lead author of the ARCH study and Professor of Medicine at Columbia University, stated: "For the first time, we have evidence that starting with an anabolic agent followed by an antiresorptive produces significantly fewer fractures than starting with an antiresorptive alone. This challenges the traditional treatment sequence for high-risk patients."

The ARCH trial also revealed the cardiovascular signal that would complicate the drug's regulatory path. Adjudicated serious cardiovascular events (myocardial infarction, stroke, cardiovascular death) occurred in 2.5% of romosozumab patients versus 1.9% of alendronate patients during the 12-month double-blind period. The difference was not statistically significant in isolation, but FDA reviewers considered it clinically concerning given the drug's intended population.

The Regulatory Journey: From Rejection to Approval (2016 to 2019)

Amgen and UCB submitted the first Biologics License Application (BLA) to the FDA in July 2016, based primarily on FRAME data. ARCH results were still maturing at that time.

In July 2017, the FDA issued a Complete Response Letter, declining to approve romosozumab. The agency requested additional analysis of the cardiovascular safety signal that emerged from ARCH, plus resolution of manufacturing-related observations from a facility inspection 9.

Amgen resubmitted the BLA in July 2018 with updated cardiovascular analyses, a Risk Evaluation and Mitigation Strategy (REMS) communication plan, and manufacturing remediation. The FDA's Bone, Reproductive and Urologic Drugs Advisory Committee voted 18-1 in January 2019 to recommend approval, acknowledging that the fracture reduction benefits outweighed cardiovascular risks in the proposed high-risk population.

On April 9, 2019, the FDA approved Evenity for the treatment of osteoporosis in postmenopausal women at high risk for fracture, defined as having a history of osteoporotic fracture, multiple risk factors for fracture, or failure or intolerance of other osteoporosis therapies. The label carried a boxed warning regarding potential increased risk of myocardial infarction, stroke, and cardiovascular death, with a contraindication in patients who have had a myocardial infarction or stroke within the preceding year.

International Approvals and Label Differences

Japan approved romosozumab in January 2019, slightly before the FDA, for osteoporosis in patients at high fracture risk. The Japanese label (marketed as Evenity) notably does not include a boxed cardiovascular warning, reflecting different risk-benefit assessments by the Pharmaceuticals and Medical Devices Agency (PMDA).

The European Medicines Agency (EMA) approved romosozumab in December 2019 for severe osteoporosis in postmenopausal women at high risk of fracture with no history of myocardial infarction or stroke 10. The EMA went further than the FDA by making prior cardiovascular events a contraindication rather than just a boxed warning consideration.

South Korea, Australia, Canada, and Brazil followed with approvals between 2019 and 2021. By 2022, romosozumab was approved in over 70 countries.

Post-Approval Evidence and Clinical Positioning (2019 to Present)

Real-world evidence has accumulated since approval. The BRIDGE study (2018) demonstrated efficacy in men, showing 12.1% lumbar spine BMD increase at 12 months in men with osteoporosis (N=245), supporting subsequent label expansions in some markets 11.

A 2022 meta-analysis of cardiovascular events across all Phase III romosozumab trials (N=11,875) found a pooled relative risk of 1.39 (95% CI 0.88-2.18) for major adverse cardiovascular events versus non-romosozumab comparators 12. The confidence interval crossing 1.0 suggests the signal remains uncertain, but clinicians have adopted a cautious approach: most avoid prescribing romosozumab in patients with established cardiovascular disease or significant risk factors.

The Endocrine Society's 2020 clinical practice guideline positions romosozumab as a first-line option for postmenopausal women at very high fracture risk, recommending anabolic-first sequencing (romosozumab for 12 months followed by a bisphosphonate or denosumab) over traditional antiresorptive-first approaches 13. The American Association of Clinical Endocrinology (AACE) 2020 guidelines similarly recommend romosozumab for patients with very high fracture risk, defined as T-score ≤ -3.0, prior vertebral fracture, or fracture within the prior 12 months.

The Sclerostin Pathway: Ongoing Research and Next-Generation Agents

Romosozumab's commercial and clinical success validated sclerostin as a drug target, spurring additional research. Several questions remain active areas of investigation.

Re-treatment studies are examining whether a second 12-month course of romosozumab produces comparable BMD gains after an intervening antiresorptive period. Preliminary data from extension studies suggest partial but meaningful response to retreatment, with smaller BMD gains than the initial course 14.

Combination approaches pairing romosozumab with denosumab during the active treatment phase (rather than sequentially) showed additive BMD effects in a small Phase II study, though this remains investigational.

Amgen has explored next-generation sclerostin pathway modulators, including bispecific antibodies targeting both sclerostin and DKK1 (another Wnt pathway inhibitor). These remain preclinical. UCB's pipeline includes antibodies targeting additional bone biology pathways identified through the sclerostin research program.

The broader scientific legacy extends beyond drug development. Romosozumab's success demonstrated that genetic observations in rare skeletal diseases can be directly translated into therapies for common conditions within two decades of target identification. The timeline from SOST gene discovery (2001) to FDA approval (2019) represents one of the more efficient bench-to-bedside translations in musculoskeletal medicine.

Manufacturing and Administration

Romosozumab is produced using Chinese Hamster Ovary (CHO) cell culture, standard for monoclonal antibody manufacturing. Each dose of 210 mg requires two subcutaneous injections of 105 mg/1.17 mL, administered in separate sites (abdomen, thigh, or upper arm). The drug is supplied in single-use prefilled syringes and stored refrigerated at 2°C to 8°C.

The requirement for two injections per monthly visit (due to volume constraints of subcutaneous delivery) was identified in patient surveys as a barrier to adherence. Amgen has explored higher-concentration formulations but has not announced an approved single-injection version as of 2026.

Treatment duration is fixed at 12 monthly doses. Bone density measurements at months 6 and 12 confirm response. After the 12-dose course, transition to an antiresorptive is mandatory. Without sequential antiresorptive therapy, BMD gains reverse within 12 months, returning to near-baseline levels. This makes romosozumab a bridge therapy rather than a standalone treatment.